Conductive pastes are frequently used as means for forming conductive circuitry and electrodes at the surface, interior or exterior of various substrates. In this specification, the term “conductive paste” indicates a fluid possessing fluidity that is generally obtained by dispersing a conductive powder as a filler (hereinafter called a “conductive filler”) in a vehicle composed of a resinous binder and a solvent and that when heated to an appropriate temperature undergoes vaporization/decomposition of the vehicle and sintering of the remaining conductive filler into a body constituting a good conductor of electricity. In other words, a paste that forms a conductor when sintered at high temperature is called a conductive paste for short. In actual use, such a conductive paste is applied to the surface or charged into an interior opening of a substrate and is subjected to a suitable heat treatment together with the substrate. The heat treatment vaporizes, decomposes and burns the vehicle, and the particles of the metallic powder constituting the conductive filler are sintered together to form a circuit capable of passing electricity. Likewise in the case of a laminated ceramic capacitor, conductive paste for the internal electrodes is interposed among a large number of ceramic substrates, conductive paste for the external electrodes interconnecting-the internal electrodes is applied, and heat treatment is similarly conducted to vaporize, decompose and remove the vehicle and sinter the metallic powder to form internal electrodes and external electrodes. At this time, the internal electrodes and external electrode are usually separately sintered.
The conductive filler (metallic powder) used in such a conductive paste is ordinarily copper powder or silver powder. Conductive pastes using copper powder as the conductive filler (copper-system pastes) have recently come into wide general use for the reason that, in comparison with conductive pastes using silver powder as the conductive filler (silver-system pastes), they are less susceptible to migration, superior in soldering resistance and able to achieve low cost. A copper-system paste with these merits is obtained by dispersing copper powder of a particle diameter of around 0.1-10 μm in an appropriate vehicle (usually composed of resin binder and a solvent).
Even among copper-system pastes, in the case of those used for the external electrodes of laminated ceramic capacitors and those for forming various circuitry on substrates, the physical and chemical properties required of the conductive paste differ with difference in electrode or circuit form, method of forming the same, substrate material, and the like. Since the general practice has therefore been to prepare copper-system pastes with various capabilities separately for each use, differences in the optimum range of coating conditions and sintering conditions are present among the individual types of copper-system pastes.
Except in certain special cases, a copper-system paste is usually desired to be sinterable at low temperature. This is because when conductive circuitry can be sintered by low temperature heating at the substrate surface and interior, the temperature to which the substrate heated together with the conductive paste is raised can be kept down to mitigate heat-related effects on the substrate, advantages can be enjoyed from the aspects of heat energy and equipment, and occurrence of strain owing to difference in thermal expansion between the ceramic substrate and copper circuitry can be reduced.